The reaction layer of a fuel cell is formed between a proton exchange membrane and a backing layer and supports a catalyst for accelerating an electrochemical reaction. In the reaction layer, on an air cathode side for example, protons passing through the proton exchange membrane and electrons transferred to the air cathode are conducted to the catalyst, for reaction between the oxygen and protons diffused onto the catalyst. Thus, the reaction layer should exhibit both proton conduction and electron conduction in order to minimize the transfer loss of oxygen, protons and electrons. To this end, a mixture of poly electrolytes having catalysts supported on their surfaces, e.g. carbon particles (exhibiting electron conduction) and Nafion (trade name, manufactured by E.I du Pont de Nemours) which exhibits ionic conduction have been used in a fuel cell (see FIG. 1B).
However, if a material having ionic conduction and a material having electronic conduction are used together, it is difficult to mix them so as to achieve complete uniformity. As a result, protons and electrons cannot be uniformly transferred to all catalyst particles.
To solve the foregoing problem, there have been proposed a variety of mixed conductors for use as carriers designed for supporting catalysts, which carriers exhibit both ionic conduction and electron conduction using one material (A carrier designed for supporting a catalyst will be hereinafter referred to as a “catalyst-supporting carrier”).
Organic catalyst-supporting carriers are disclosed in JP2001-202971A, JP2001-110428A, JP2003-68321A and JP2002-536787A. However, since the organic catalyst-supporting carriers are made of organic materials, they present many problems in terms of durability and heat resistance which are obstacles to practical use.
In addition, inorganic catalyst-supporting carriers which conduct electrons and oxygen ions are disclosed in JP1998-255832A, JP1999-335165A, JP2000-251533A, and JP2000-18811A. However, the inorganic catalyst-supporting carriers which transfer electrons and oxygen ions have high operating temperatures (about 800° C.). Due to such high operating temperatures, these inorganic catalyst-supporting carriers are inappropriate for use in small-sized fuel cells, for example in vehicles and cellular phones.
Prior to the present invention, no catalyst-supporting carrier exhibiting both proton conduction and electron conduction had been developed which operates within a moderate temperature range (room temperature to 200° C.) and which can be used in fuel cells.